Try by adding single quotes.
string sql = "select * from tblVehicle where CAST(floor( CAST( Vehentrytime AS FLOAT ) )AS DATETIME) = '" + myDate + "'";
Couple of more points -
1) Not sure why you are using
CAST ... AS FLOAT
.
2) Not sure why you are using
floor
.
3) Plain concatenated Queries may be risky considering "
SQL Injection", instead use Parametrized Queries.